Newton related his ideas to Leibnitz in letters. Leibnitz developed the ideas, and published, but neglected to give Newton proper credit. That was the source of the conflict.
"Newton related his ideas to Leibnitz in letters. Leibnitz developed the ideas, and published, but neglected to give Newton proper credit. That was the source of the conflict."
You're wrong. The claim is that Leibnitz may have seen some of Newton's unpublished manuscripts -- that while not spelling out calculus, would have given direction, hints.
Leibnitz admitted he had seen some of N's MS. And recently, IIRC, one of N's MS was found amongst L's possessions. But that still doesn't prove Leibnitz plagiarized. There was not enough in the MS for it to be plagiarism.
Leib published his calculus 20 years before Newton's came out (in the Optiks). It's hard to believe L got it all out of some MS that wasn't even on that subject, but at best suggestive of the subject.
No, it's far more likely that it was in the air--the zeitgeist. A lot of other people had been and still were working on the same stuff at the same time. These things happen.